Our studies of adult immune reconstitution have demonstrated that severe deficits in naive T cells and TCR repertoire develop and persist in older patients with limited renewal of thymopoiesis. In order to develop IL-7 as a potential therapeutic agent to enhance nave populations in these patients, we initiated the first phase I study of recombinant human IL-7 (rhIL-7) administration in humans. We demonstrated that two weeks of alternate day treatment with rhIL-7 produced a marked dose-dependent increase in the numbers of circulating CD4+ and CD8+ T cells that persisted in follow-up assays at 6 to 12 weeks post treatment.14,15 Furthermore, rhIL-7 therapy disproportionately increased CCR7+CD27+CD45RA+ naive and CCR7+CD27+CD45RA- central memory cells, which represent the most diverse components of the mature TCR pool, at the expense of the CCR7-CD27-CD45RA+/- effector populations. The proportion of naive cells in the total CD8+ population increased by as much as 39%. We further documented that IL-7 produced a prolonged period of cellular expansion (Ki67+ ) and elevation of anti-apoptotic factors (Bcl-2) in naive and memory T cells, but not in effector T cells. Part of the basis for this disparity is the relatively low expression of the IL-7R(CD127) in effector T cells, particularly CD8 effectors. Similarly Treg cells, which have low expression of IL-7R, did not show the same sharp increase in the percentage of cells in cycle following initiation of IL-7 therapy and declined as a percentage of the total CD4 population. Because of the extent of this population shift, we hypothesized that IL-7 would lead to an overall increase in TCR diversity in CD4+ and CD8+ T-cells. We assessed TCR diversity using spectratype analysis on sorted CD4 and CD8 populations at day 0 and one week after rhIL-7 therapy (day 21) in six subjects. Three of these subjects were over 60 years of age, and a fourth patient was severely T cell deficient following recent chemotherapy. For each patient, we compared pre- and post-therapy spectratype divergence from a Gaussian-like normal donor standard. The global diversity (divergence from a normal donor standard in each of 22 BV families) of pre and post spectratypes was compared by Wilcoxon paired non-parametric analysis. We determined that 4 of the 6 subjects had a statistically significant increase (P &lt;.05) in repertoire diversity following IL-7 treatment, as compared to baseline, in either the CD4+, CD8+, or both T-cell populations. This expansion of nave and central memory T cells and the disproportional loss in effector cells was particularly evident in CD8 populations in which 5/6 patients had either a significant shift or a strong trend toward increased repertoire diversity. Given the short duration of therapy, the advanced ages of some patients, and the PCR-assessed decline in the frequency of TREC in even the most nave T cells (sorted CD31+CD45RA+ CD4 cells) that we observed, this enhancement in diversity was due primarily to differential population expansion, not IL-7 induced thymic output. We have thus shown that rhIL-7 has the potential to induce thymic-independent T-cell growth in naive and CM populations and enhance repertoire diversity in peripheral T-cell populations. We have also initiated a new clinical trial to treat the pulmonary complication of chronic graft versus host disease known as bronchiolitis obliterans. Preliminary results are encouraging and the study remains open and active. A new trial has also begun to treat leukemia with myeloablative therapy and assess improvement in immune reconstitution by modulation of thymus function.